Abnormality detection devices have previously been proposed which can detect abnormalities of insulation resistance of high-voltage circuits. Patent Literature 1 discloses a ground-fault detection circuit, as an abnormality detection device, used in an electric vehicle. The ground-fault detection circuit disclosed in Patent Literature 1 includes: a coupling capacitor, an oscillator circuit unit, and a ground-fault detection circuit unit. The coupling capacitor has one end that is coupled with a high-voltage circuit. The oscillator circuit unit outputs an oscillation signal to the other end of the coupling capacitor via impedance. The ground-fault detection circuit unit determines occurrence of grounding of the high-voltage circuit on the basis of the amplitude of a signal between the impedance and the coupling capacitor.
In accordance with the ground-fault detection circuit disclosed in Patent Literature 1, the oscillation signal that is output from the oscillator circuit unit can be detected by the ground-fault detection circuit unit without a large voltage drop, as long as insulation resistance of the high-voltage circuit is normally held. On the other hand, if a ground fault develops in the high-voltage circuit, the oscillation signal that is output from the oscillator circuit unit exhibits a large voltage drop due to an electric current that flows from the high-voltage circuit to a ground point. The ground-fault detection circuit unit detects the voltage drop. This configuration makes it possible to determine whether or not a ground fault develops in the high-voltage circuit.
Moreover, the ground-fault detection circuit disclosed in Patent Literature 1 is equipped with either a band-pass filter or a high-pass filter between the ground-fault detection circuit unit and the coupling capacitor. The filter interrupts low-frequency noises generated in the high-voltage circuit. Unless the low-frequency noises are interrupted, the oscillation signal from the oscillator circuit unit is added with the low-frequency noises, then is inputted to the ground-fault detection circuit unit. For this reason, the thus-inputted signal may exceed a dynamic range of the ground-fault detection circuit unit. In this case, a part of the signal inputted to the ground-fault detection circuit unit is unfavorably clipped off, making it difficult to correctly detect ground faults. Fortunately, the presence of either the band-pass or high-pass filter described above allows avoidance of such a signal clipping.